In electrolysis nowadays, particularly in zinc electrolysis, cathode plates made of aluminium are used, which are connected to support bars. The cathode is lowered into the electrolysis cell by the support bars so that one end of the support bars is located on top of the busbar at the edges of the cell and the other end on top of the insulation. To ensure good electrical conductivity, a contact piece made of copper is attached to the ends of the aluminium support bar, and the contact piece is set on top of the busbar. The lower edge of the contact piece is either horizontal or a notch is made there and the support bar is lowered on top of the busbar at the notch. Both side edges of the notch form a linear contact, creating a double contact between the support bar and the busbar. When the lower edge of the contact piece is straight, an plane-type contact is formed between the busbar and contact piece. A contact piece of this kind is used particularly in large cathodes, known as jumbo cathodes.
The copper contact piece can be attached to the aluminium support bar for example by various welding methods. One of these methods is described for instance in U.S. Pat. No. 4,035,280. The Japanese application 55-89494 describes another method of manufacturing an electrode support bar. The actual support bar is aluminium and to its end is welded a contact piece with an aluminium core and a copper shell. The contact pieces are given their polygonal form using high-pressure extrusion.
When copper is joined to aluminium, brittle and poorly conductive phases, such as Al2Cu, AlCu, Al3Cu4, Al2Cu3 and AlCu3, can easily be formed on the interface. These phases contain non-metallic covalent bonds and it is these that give rise to their great electrical resistance. Generation of these phases is possible for instance during fusion welding. Diffusion-based jointing methods may also cause the generation of the above-mentioned phases.
The tendency of aluminium to form a passivation layer on its surface i.e. a thin oxide film, in the presence of air or moisture, is a great hindrance to the joining of aluminium to other materials e.g. using soldering methods, and also to the fabrication of aluminium-aluminium joints. This is, in fact, the greatest single problem in joining copper and aluminium to each other. The passivation layer prevents contact between the metal and the solder, and thus when using brazing technique the oxide film has to be removed before brazing. One can attempt to remove the oxide film before preparing the joint, but the oxidation reaction is very quick and in an air atmosphere the formation of oxide cannot be avoided. There are also what are termed active solders on the market, which are claimed to moisten the aluminium regardless of the oxide layer, but their alloying elements, however, are not suitable for an electrolysis environment. In addition, solders that melt at low temperatures i.e. below 250° C., have to be stripped away, because the temperature of the contact pieces may in exceptional circumstances (short circuits) rise quite high locally and this limits the use of said solders in electrolysis.
DE patent application 3323516 describes a method in which cathodes are used in zinc electrolysis, where the support bar is aluminium and the copper contact pieces are attached to it by soldering. The solder used is an aluminium/silicon-based solder.
In the research that we carried out it was found that the use of aluminium rods containing silicon in aluminium and copper welding generates Al—Si eutectic, which fare badly in the corrosive conditions of electrolysis.
As stated before, achieving a good connection between copper and aluminium is difficult. The electric current passing via the contact pieces to the cathode can nevertheless be considerable, e.g. in the range of 600-1600 A. If the joint between the actual support bar and the contact piece in the electrode support bar is poor, the current travels only locally in the joint and the current flowing through these points becomes excessively large per unit of surface area. This causes local overheating and as a result the oxidation of copper, which further worsens the flow of the current to the cathode.
U.S. Pat. No. 4,035,280 also mentions that copper contact pieces can be coated with silver before welding. It is clear that a silvered contact piece conducts electricity well, but if the welding joint between the aluminium support bar and the contact pieces remains poor, that is a more decisive factor on the whole than the use of silver in the contact pieces.